Control equipment for active filters and a method for reduction of harmonics in a bipolar dc link

ABSTRACT

A bipolar converter station for conversion between alternating current and high-voltage direct current comprises a first and a second active filter (AF11, AF12), each with its own piece of control equipment (CE11, CE12). The filters are intended for reduction of harmonics in a dc link, connected to the converter station, with a first and a second pole line (PC1, PC2), and each filter generates, in dependence on a sum of harmonic currents in the respective pole line, an influencing quantity (IF11, IF12) and supplies this to the dc link. The control equipment comprises frequency-selecting means (BP, CTR) which influence the frequency content of the respective influencing quantity to lie substantially within one of a first and a second of at least two predetermined, mutually non-overlapping frequency bands (FR1, FR2). During bipolar operation of the converter station, the frequency-selecting means in the control equipment for the first active filter influence the frequency content of the influencing quantity, generated by this filter, to lie within the first frequency band and the frequency-selecting means in the control equipment for the second active filter influence the frequency content of the influencing quantity, generated by this filter, to lie within said second frequency band.

TECHNICAL FIELD

The present invention relates to a pair of first and second controlequipment for a first and a second active filter, respectively, in abipolar converter station for conversion between alternating current andhigh-voltage direct current, the filters being intended for reduction ofharmonics in a dc link connected to the converter station, and to amethod for reduction of harmonics in a dc link, connected to a bipolarconverter station, by means of a first and a second active filter.

Each one of the pieces of control equipment comprisesfrequency-selecting means which influence the frequency content of aninfluencing quantity, generated by the respective active filter, toessentially lie within one of a first and a second of at least twopredetermined, mutually non-overlapping frequency bands.

BACKGROUND ART

A converter for conversion between alternating current and high-voltagedirect current, connected between an ac link and a dc link, generates,through its mode of operation, harmonics in the current in the acnetwork and in the voltage of the dc link. The currents associated withthe latter voltage harmonics, and in particular the so-calledground-mode current, constitute a source of disturbance fortelecommunication equipment located in the vicinity of the dc link. Toeliminate these disturbances to the greatest possible extent, passiveshunt filters are often used in the dc link, tuned to frequencies whichare integer multiples of a product of the pulse number of the converterand the system frequency in the ac network, as well as designed ashigh-pass filters. These filters are generally not capable of completelyextinguishing harmonics in the dc link. The reasons therefor are, forexample, drift in the system frequency and in component values in thefilters, but also the fact that the filter impedance at resonancefrequency is not always negligible compared with the impedance in theother parts of the system. Further, during commutations and because ofphase asymmetries in the ac network, harmonics of other frequencies thanthose mentioned above are normally also generated.

The passive filters are therefore nowadays often supplemented withactive filters, which supply to the dc link an influencing quantity inthe form of a current or a voltage for the purpose of reducing theamplitude of the harmonics which are generated by the phenomenadescribed above. By measuring the remaining harmonic content in thedirect current, the supplied current or voltage may thus be given such aform that, in principle, it fully eliminates the harmonic content. Sucha filter comprises a power amplifier which is controlled by controlequipment which is supplied with a sensed value of the harmonics,usually an actual current value formed in dependence on the harmoniccurrent in the dc link. The control equipment forms, in dependence on adifference of a reference value for the harmonics and their sensedvalue, a control signal which is supplied to the power amplifier. Independence on the control signal, the power amplifier generates theinfluencing quantity and supplies this to the dc link as one of a seriesquantity or a shunt quantity. Such filter circuits are exemplified in L.Gyugyi and E. C. Strycula: Active AC Power Filters, IAS 76 Annual, Paper19-C. The document describes filter circuits for ac networks, butcircuits which, in principle, are similar may also be applied to a dclink. In practice, the influencing quantity is usually supplied as ashunt quantity in that the power amplifier is connected in parallel withthe disturbance source, in this case the converter.

The control signal is formed by a controller, which usually has aproportional or a proportional/integrating characteristic, whichcontroller is supplied with the above-mentioned difference of thereference value for the harmonics and their sensed value.

An embodiment of such a controller is described in the publishedinternational patent application WO 93/09585. The controller forms thecontrol signal in the form of a pulse train, repetitively with a cycletime corresponding to the repetitiveness of the disturbance source, inthis case the time between two commutations in the converter, andcomprises delay filters for adaptation of the controller to time delaysin the transfer function for the external circuit connected to thecontroller.

The controller may also comprise filter devices for reducing oreliminating, in its output signal, certain frequency components orfrequency bands, for example within a lower frequency range, thedisturbing influence of which on the surroundings is low but whichrequire a high power output from the power amplifier to be able to beeliminated.

The unpublished Swedish patent application 9700897-3 describes controlequipment for an active filter, comprising at least one, and generally aplurality of, control units arranged like each other, each one forreduction of a tone of a tone frequency nω of the ac components of thedirect current. In this connection, tone means an interference signal,for example a harmonic current, of the tone frequency nω, where ωdesignates the system frequency of the alternating-voltage network andn, the ordinal number of the tone, a real number separate from zero,preferably an integer, n=n₁, n₂, . . . n_(m). Especially ininstallations for transmission of high-voltage direct current, where theconverters included, because of their mode of operation, generate, ontheir direct-voltage sides, harmonics of the ordinal numbers q=kp to thesystem frequency of the alternating-voltage network, where p is thepulse number of the converter and k is a positive integer, the ordinalnumbers may be chosen as integer multiples of the pulse number of theconverter.

Each one of the control units is supplied with the actual current valueand generates, in dependence thereon, an output signal. The controlsignal for the active filter is generated in dependence on this outputsignal, or in the event that the control equipment comprises a pluralityof control units, in dependence on the sum of the outputs signalsthereof, whereby a simultaneous reduction of the amplitude of a numberof current harmonics of the tone frequencies nω=n₁ ω, n₂ ω, . . . n_(m)ω is achieved.

Each one of the control units comprises a first frequency-transformingmember which forms values of amplitude and phase position for the tonewhich is associated with the respective control unit. These values arethen processed separately in a separate controller withproportional/integrating characteristic, whereupon the output signalsfrom the two controllers are summed and an output signal with afrequency equal to the tone frequency associated with the controller isrecreated. The transfer function of the control unit is adapted, withrespect to amplitude and phase position, to the transfer function forthe external circuit connected to the control unit.

A converter station intended for bipolar operation essentially comprisestwo converters, which are series-connected on the dc side, andgenerates, in relation to ground, a direct voltage with positivepolarity and a direct voltage with negative polarity (FIG. 1). Theinterconnecting point between the converters is connected to ground viaan electrode line and a dc link, comprising two pole lines, is energizedwith the positive and the negative voltage, respectively. During bipolaroperation, the current through the electrode line is zero or near zeroand the power is transmitted via the pole lines. Only in exceptionalcases, in case of a fault on or maintenance of a pole, the station isused in unipolar operation, in which case the dc circuit consists of oneof the pole lines and the ground connection to an additional stationconnected to the pole line.

In converter stations of the above kind, an active filter is usuallyconnected between each of the pole lines and ground, whereby the purposeof the control of the filters is to reduce the harmonic content in theground-mode current. Each one of the pieces of control equipment of thefilters is adapted to form the control signal in dependence on a sum ofthe harmonic currents of both pole lines, with the purpose of reducingthis sum of harmonic currents. Studies have shown that, particularly inthis operating case, where thus both pieces of control equipment aresupplied with the same actual current values, interference phenomena mayarise between the filters, characterized in that each one of the filtersgenerates influencing quantities resulting in harmonic currents with acertain frequency content, which in turn the second filter strives toreduce by forming an influencing quantity with a corresponding frequencycontent. This phenomenon leads to an unstable control of the harmoniccontent in the dc link.

SUMMARY OF THE INVENTION

The object of the invention is to achieve a method of the kind mentionedin the introductory part of the description, which allows a stable andefficient reduction of harmonics in the ground-mode current duringbipolar operation of the converter station, as well as a device forcarrying out the method.

According to the invention, this is achieved in that each piece ofcontrol equipment comprises frequency-selecting means which influencethe frequency content in an influencing quantity, generated by therespective active filter, to substantially fall within one of a firstand a second of at least two predetermined, mutually non-overlappingfrequency bands.

Advantageous improvements of the invention will become clear from thefollowing description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail by description ofembodiments with reference to the accompanying drawings, which are allschematic and in the form of single-line diagrams and block diagrams,respectively, and wherein

FIG. 1 shows a bipolar installation for transmission of high-voltagedirect current with active filters connected to a dc link,

FIG. 2 shows a bipolar converter station in an installation according toFIG. 1,

FIG. 3 shows an embodiment of a deviation-forming unit comprisingfrequency-selecting means according to the invention, in a converterstation according to FIG. 2,

FIG. 4 shows an embodiment of a deviation-forming unit and a controlunit comprising frequency-selecting means according to the invention, ina converter station according to FIG. 2,

FIG. 5 shows an embodiment of control equipment comprisingfrequency-selecting means according to the invention, in a converterstation according to FIG. 2,

FIGS. 6A-6D illustrate the frequency content of influencing quantitiesgenerated by active filters with a pair of control equipment accordingto the invention, in an embodiment of an installation according to FIG.1, and

FIGS. 7A-7D illustrate the frequency content of influencing quantitiesgenerated by active filters with a pair of control equipment accordingto the invention, comprising single-tone controllers, in an embodimentof an installation according to FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description relates both to the method and to the device.

The block diagrams show measured values and diagrams for forming certaincalculating values which are used in other blocks shown. Interconnectinglines between these measured values and these blocks have in certaincases been omitted in order not to burden the drawings, but it is to beunderstood that the respective calculating values are fetched from thoseblocks in which they are formed and that measured values are formed insome way known per se by sensing of the corresponding quantities in theinstallation for transmission of high-voltage direct current.

Further, it is to be understood that, although the blocks shown in thefigures are referred to as units, members, filtering members, etc.,these are to be understood as means for achieving the desired functions,especially in those cases where their functions are implemented assoftware in, for example, microprocessors.

FIG. 1 illustrates, in the form of a single-line diagram, a knownconfiguration of a bipolar installation for transmission of high-voltagedirect current. A bipolar converter station comprises a converter CV11and a converter CV12, each being connected on its ac side to an acnetwork N1. The converters are of a known design and, although notspecifically shown in the figure, for example arranged in a 12-pulseconnection. Converter transformers, ac filters and other ac apparatusconventionally arranged for such a converter station are assumed to becomprised in the ac network. The converters are series-connected ontheir dc sides by means of two neutral conductors NC11 and NC12,respectively, and the interconnecting point between the neutralconductors is connected, via an electrode line EL1, to a groundingelectrode GE1. The converter CV11 is connected, by way of a smoothinginductor LD11, to a first pole line PC1 and the converter CV12 isconnected, by way of a smoothing inductor LD12, to a second pole linePC2. A current ICV11 flows through the converter CV11 and a currentICV12 flows through the converter CV12. Between a connection point J111on the pole line PC1 and a connection point J112 on the neutralconductor NC11, a series circuit comprising a passive filter PF11 and afirst active filter AF11 is connected. The current I1 in the pole lineconsists of a dc component and a harmonic current and is sensed with afirst current-sensing member AV11, which as output signal SI1 forms anactual current value in dependence on the harmonic current. The actualcurrent value SI1 is supplied to a first piece of control equipmentCE11, which, in dependence thereon, forms a control signal SC11 for thefirst active filter. In dependence on the control signal, the activefilter forms an influencing quantity, in this embodiment of theinstallation in the form of a filter current IF11, which, at theconnection point J111, is added to the current ICV11 through theconverter. The current I1 in the pole line PC1 thus consists of the sumof the two currents ICV11 and IF11.

A series circuit comprising a passive filter PF12 and a second activefilter AF12 is connected between a connection point J121 on the poleline PC2 and a connection point J122 on the neutral conductor NC12. Asecond current-sensing member AV12 forms, in a manner similar to what ismentioned above, an actual current value S12 in dependence on a harmoniccurrent in the pole line PC2, which actual current value is supplied toa second piece of control equipment CE12, which, in dependence thereon,forms a control signal SC12 for the second active filter AF12. Independence on the control signal SC12, the second active filter forms aninfluencing quantity, in this embodiment of the installation in the formof a filter current IF12, which at the connection point J121 is added tothe current IC12 through the converter. The current I2 in the pole linePC2 thus consists of the sum of the two currents ICV12 and IF12.

FIG. 1 further shows an additional bipolar converter station comprisinga converter CV21 and a converter CV22, each one on their ac sidesconnected to an ac network N2. The converter CV21 is connected, via asmoothing inductor LD21, to the first pole line PC1 and the converterCV22 is connected, via a smoothing inductor LD22, to the second poleline PC2. The converters are connected in series on their dc sides bymeans of two neutral conductors NC21 and NC22, respectively, and theinterconnecting point between the neutral conductors is connected to agrounding electrode GE2 via an electrode line EL2. Otherwise, thisconverter station is composed in the same way as the one describedabove, and the description above is applicable when, in referencenumerals for corresponding blocks, currents and signals, index 11 isreplaced by index 21 and index 12 by index 22.

The system frequency of the ac network is, in the following, designatedω.

In the embodiment of the installation according to FIG. 1, the pieces ofcontrol equipment for the active filters are adapted for reduction ofthe harmonic content in the ground-mode current, which, with referencedirections for the currents in the pole lines as chosen according to thefigure, is defined as half the sum of these currents. All the pieces ofcontrol equipment are therefore supplied with both the actual currentvalue SI1 and the actual current value SI2 and are adapted to form thecontrol signal in dependence on the sum of these two actual currentvalues.

Conventionally, the pieces of control equipment for the active filters,in an installation as the one described with reference to FIG. 1, arearranged such that each one of the active filters generates aninfluencing quantity with a frequency content corresponding to thefrequency content in the harmonic current. As mentioned above, certainfrequency components or frequency bands are possibly suppressed, forexample within a lower frequency range, to reduce the power output fromthe power amplifier. For this purpose, these frequency components orfrequency bands are then suppressed in a similar manner for all theactive filters included in the installation.

According to the invention, each piece of control equipment is nowadapted to comprise frequency-selecting means influencing the frequencycontent of the respective influencing quantity to essentially lie withinone of a first and a second of at least two predetermined, mutuallynon-overlapping frequency bands. During bipolar operation of theconverter station, the frequency selecting means in the controlequipment for the first active filter are adapted to influence thefrequency content of the influencing quantity generated by this filterto lie within the first frequency band and the frequency-selecting meansin the control equipment for the second active filter to influence thefrequency content of the influencing quantity generated by this filterto lie within the second frequency band.

This is illustrated in FIGS. 6A and 6B, the horizontal axes of which aregraded in frequency ω. In FIG. 6A, where the designation FC/IF11 meansfrequency content in the influencing quantity generated by the firstactive filter, that is, in the embodiment of the installation describedwith reference to FIG. 1, in the filter current IF11, a first frequencyband FR1 is shown, and in FIG. 6B, where the designation FC/IF12 meansfrequency content in the influencing quantity generated by the secondactive filter, that is, in this case in the filter current IF12, asecond frequency band FR2 is shown. The figures also illustrate the factthat the two frequency bands mentioned are chosen so as not to overlapeach other.

FIG. 2 shows in more detail the pieces of control equipment for theactive filters in a bipolar converter station in an installationaccording to FIG. 1. Otherwise, FIG. 2 corresponds to the lefthand partof FIG. 1 and elements with the same reference numerals are of the samekind in both figures. Each one of the pieces of control equipment CE11and CE12 comprise a deviation-forming unit DEVU, which is supplied withthe actual current values SI1 and SI2 (in this embodiment it is assumedthat the active filters, as mentioned above, are adapted for reductionof harmonics in the ground-mode current). The respectivedeviation-forming unit forms a deviation signal, in the figuredesignated SD11 and SD12, respectively, in dependence on the differenceof a reference value signal REF (not shown in this figure) for theharmonics and the sum of the actual current values. Each one of thepieces of control equipment further comprises a control device CTR,which is supplied with the respective deviation signal and, independence thereon, forms a control signal, in the figure designatedSC11 and SC12, respectively, for respectively the first and the secondactive filter.

FIG. 3 illustrates an embodiment of the invention in which thefrequency-selecting means are comprised in the deviation-forming unit.The actual current values SI1 and SI2 are summed in a summing member 21which forms a sum actual value SSI. The sum actual value is supplied toa band-pass filtering unit BP, which, in some manner known per se, isadapted to forward, as output signal SSB, those components of thesupplied signal which have a chosen frequency content, for example thosewhich lie within the above-mentioned first frequency band, but to blockthe other components. This is schematically illustrated in the figureby--in this embodiment--three band-pass filtering members BP1, BP2 andBP3, respectively, comprised in the band-pass filtering unit. Each oneof the band-pass filtering members is adapted to forward, in some mannerknown per se, in its output signal those components of the suppliedsignal which lie within a certain frequency band, but to block othercomponents. Each one of these members is supplied with the sum actualvalue via a switching member SW1, SW2 and SW3, respectively, which maybe influenced in dependence on an order signal FSEL. A summing member 23is supplied with the output signals from all the band-pass filteringmembers and forms the output signal SSB from the band-pass filteringunit as the sum of the latter output signals.

The output signal SSB from the band-pass filtering unit and theabove-mentioned reference-value signal REF are supplied to a summingmember 22, which as output signal forms the deviation signal SD as thedifference of the reference value signal and the output signal SSB. Thereference value signal is preferably given the value zero.

For example, the band-pass filtering member BP1 in the first piece ofcontrol equipment is adapted to forward components with a frequencycontent within the first frequency band and the band-pass filteringmember BP2 in the second piece of control equipment is adapted toforward components with a frequency content within the second frequencyband. If, in the first piece of control equipment, the switching memberSW1 is closed whereas the other switching members are open, and in thesecond piece of control equipment the switching member SW2 is closedwhereas the other switching members are open, the first active filterwill thus, on the first pole line, supply an influencing quantity,generated by this filter, with a frequency content which is within thefirst frequency band and the second active filter will supply, on thesecond pole line, an influencing quantity, generated by this filter,with a frequency content which is within the second frequency band.

FIG. 4 illustrates an embodiment of the invention in which thefrequency-selecting means are comprised in the control device CTR. Theactual current values SI1 and SI2 are supplied to a summing member 31which form the sum actual value SSI as output signal. The sum actualvalue and a reference value signal REF are supplied to a summing member32, which forms, as output signal, a deviation signal SD in dependenceon the difference of the reference value signal and the sum actualvalue. The control device comprises a plurality, in this embodimentthree, of control units RD1, RD2 and RD3, respectively, mutuallycomposed in a similar manner and of the same kind as those described inthe introductory part of the description with reference to the mentionof the unpublished Swedish patent application 9700897-3, each one forreduction of a tone of the tone frequency nω in the ground-mode current.The deviation signal, which in this embodiment thus, in principle,contains all the harmonics occurring in the currents I1 and I2, issupplied to each one of the control units. A signal-generating member 33forms, in a manner known per se, for each ordinal number n a sine signalSSIN(n)=sin(nωt) and a cosine signal SCOS(n)=cos(nωt), in dependence ona supplied value of the system frequency ω of the ac network and on anorder signal FSEL, containing information about at least one and ingeneral a plurality of ordinal numbers n for one or more tones.

Each one of the control units forms, in a known manner, for example inthe manner described in the introduction, an output signal in dependenceon the deviation signal. The output signals from all the control unitsare supplied to a summing member 34, which, as output signal, forms thecontrol signal for the active filter associated with the respectivecontrol equipment, in the figure marked with SC11 for the first activefilter, in dependence on the sum of the output signals of the controlunits.

If, in this connection, in the first piece of control equipment, sineand cosine signals corresponding to the ordinal number n=n₁ are formedand supplied to the control unit RD1, and, in the second piece ofcontrol equipment, sine and cosine signals corresponding to the ordinalnumber n=n₂ are formed and supplied to the control unit RD2, the firstactive filter will, on the first pole line, supply an influencingquantity, generated by this filter, with a frequency contentcorresponding to the frequency of the tone nω=n₁ ω and the second activewill supply, on the second pole line, an influencing quantity, generatedby this filter, with a frequency content corresponding to the frequencyof the tone nω=n₂ ω. This is illustrated in FIGS. 7A and 7B, to whichotherwise the same description as for FIGS. 6A and 6B applies. FIGS. 7Aand 7B also illustrate the fact that the two ordinal numbers mentionedare chosen so as not to coincide.

Another embodiment of the invention is shown in FIG. 5 and illustratesthe case where the frequency-selecting means are arranged at therespective current-sensing member. A band-pass filtering unit BP, whichmay be of the same kind as that described with reference to FIG. 3, issupplied with the actual current value SI1 from the current-sensingmember AV11. The output signal from the band-pass filtering unit, in thefigure marked with SI1B, and a corresponding output signal SI2B, formedin similar manner in dependence on the current I2 in the second poleline PC2, are supplied to a deviation-forming unit DEVU. In thisembodiment, this unit may be of the same kind as that described withreference to FIG. 4, with the only difference that, instead of beingsupplied with the actual current values SI1 and SI2, thedeviation-forming unit is supplied with the output signals SI1B and SI2Bfrom the respective band-pass filtering units. These band-pass filteringunits are comprised in the control equipment, but in this embodimentthey are physically located at the respective current-sensing member,which in the figure is marked with a dash-dotted line.

In an advantageous embodiment of the invention, the frequency-selectingmeans, as illustrated in FIGS. 3, 4, 6 and 7, may be designed such thateach of them influences the frequency content of the influencingquantity, generated by the respective filter, to lie within at leastalso a third predetermined frequency band FR3, not overlapping the firstor the second frequency band.

In an embodiment illustrated in FIG. 3, the band-pass filtering memberBP3 in the first control equipment is adapted to forward components witha frequency content within the third frequency band. If, in thiscontext, in the first piece of control equipment, both the switchingmember SW1 and the switching member SW3 are closed whereas the switchingmember SW2 is open, and in the second piece of control equipment theswitching member SW2 is closed whereas the other switching members areopen, the first active filter will thus, on the first pole line, supplyan influencing quantity, generated by this filter, with a frequencycontent which lies within the first frequency band and within the thirdfrequency band, whereas the second active filter supplies, on the secondpole line, an influencing quantity generated by this filter, with afrequency content which lies within the second frequency band. This isillustrated in FIGS. 6A and 6B. In FIG. 6A, the third frequency band FR3is shown, which is chosen so as not to overlap the first or the secondfrequency band, in this embodiment such that the second frequency bandis located between the first and third frequency bands.

It is realized from the above description that, by comprising in theband-pass filtering unit BP further band-pass filtering members of thesame kind as those described above, BP1, BP2 and BP3, as well asswitching members which may be influenced in dependence on the ordersignal FSEL, both the first and the second active filter may be adaptedto supply, on the respective pole lines, an influencing quantity with afrequency content which lies within a desired number of frequency bands,whereby these are chosen so as not to overlap each other. This isillustrated in FIG. 6B in that the second active filter supplies, on thesecond pole line, an influencing quantity, generated by this filter,with a frequency content which lies within a fourth frequency band FR4.

Likewise, it is realized from the above description that the controlunit, described with reference to FIG. 4, in analogous manner may beadapted to comprise further control units for an additional number oftones, such a further control unit RD3 being shown in the figure. FIGS.7A and 7B illustrate that the first active filter, supplies, on thefirst pole line, an influencing quantity, generated by this filter, witha frequency content corresponding to the frequency for a tone with theordinal number n=n₁ and the frequency for a tone with the ordinal numbern=n₃, and that the second active filter supplies, on the second poleline, an influencing quantity, generated by this filter, with afrequency content corresponding to a tone with the ordinal number n=n₂and the frequency for a tone with the ordinal number n=n₄. FIGS. 7A and7B also illustrate the fact that the four ordinal numbers mentioned arechosen so as not to coincide.

In still another advantageous improvement of the invention, a first pairof control equipment (CE11 and CE12 in FIG. 1), arranged, for example,according to any of the preceding examples, may be arranged in a firstconverter station (with converters CV11 and CV12 in FIG. 1) and a secondpair of control equipment (CE21 and CE22 in FIG. 1), of a similar kind,be arranged in a second converter station (with converters CV21 and CV22in FIG. 1). In this improvement of the invention, the filter AF11 in thefirst and the filter AF21 in the second converter station are adapted tosupply the influencing quantity, generated by the respective filter, tothat part of the dc link which comprises the first pole line and thefilter AF12 in the first and the filter AF22 in the second converterstation are adapted to supply the influencing quantity, generated by therespective filter, to that part of the dc link which comprises thesecond pole line. During bipolar operation of the converter stations,the frequency-selecting means in the control equipment for the activefilter AF11 in the first converter station and the frequency-selectingmeans in the control equipment for the active filter AF22 in the secondconverter station are adapted to influence the frequency content of theinfluencing quantity generated by the respective filter to lie withinthe first frequency band, and the frequency-selecting means in thecontrol equipment for the active filter AF12 in the first converterstation and the frequency-selecting means in the control equipment forthe active filter AF21 in the second converter station are adapted toinfluence the frequency content of the influencing quantity, generatedby the respective filter, to lie within the second frequency band. Thisis illustrated in FIGS. 6C and 6D. In FIG. 6C, where the designationFC/IF21 means frequency content in the influencing quantity generated bythe active filter AF21, that is, in the embodiment of the installationwhich is described with reference to FIG. 1, in the filter current IF21,the second frequency band FR2 is shown, and in FIG. 6D, where thedesignation FC/IF22 means frequency content in the influencing quantitygenerated by the second active filter, that is, in this case in thefilter current IF22, the first frequency band FR1 is shown. The figuresalso illustrate the fact that the two frequency bands mentioned arechosen so as not to overlap each other. Otherwise, the same descriptionapplies to FIGS. 6C and 6D as for FIGS. 6A and 6B.

FIGS. 6C and 6D also illustrate the embodiments of the invention where,for each of the pieces of control equipment, the band-pass filteringunits BP comprise additional band-pass filtering members such that, inthis embodiment, the active filter AF21 on the first pole line suppliesan influencing quantity, generated by this filter, with a frequencycontent which lies within the second frequency band FR2 and the fourthfrequency band FR4, and the active filter AF22 on the second pole linesupplies an influencing quantity, generated by this filter, with afrequency content which lies within the first frequency band FR1 and thethird frequency band FR3. FIGS. 7C and 7D illustrate correspondingconditions in the event that the frequency-selecting means consist ofsingle-tone controllers of the kind described with reference to FIG. 4.

By the above-mentioned arrangement of the pairs of control equipment,the interference between the two converter stations as well as betweenthe two poles in the respective station is minimized.

The frequency bands for the respective active filter are advantageouslychosen such that, during bipolar operation of the converter station, thepower supplied by the filters on the respective parts of the dc link isdistributed as equally as possible between the filters.

The invention is not limited to the embodiments shown but a plurality ofmodifications are feasible within the scope of the claims. For example,the band-pass filtering units may be formed in alternative ways, knownto the person skilled in the art. The pieces of control equipment may,of course, also be designed to form influencing quantities withfrequency content distributed over an arbitrary number of frequencybands.

By means of the invention, the active filters for the two poles within aconverter station may operate independently of each other, which alsoimplies that they do not need to be activated in a synchronous mannerwhen the station is taking up power.

In the event that one of the filters for some reason, for exampleoverload, must be taken out of operation, the other filter may bemaintained in operation. By forming the frequency-selecting means, insome manner known per se, so that they are capable of being influenced,the latter filter may be adapted to temporarily assume the function ofthe filter which has been taken out of operation.

The number of frequency bands/frequencies, for which a reduction of theharmonics is obtained, may be doubled while maintaining the amount ofhardware in each of the pieces of control equipment.

Any interference between two converter stations during bipolar operationmay be avoided by choosing, for filters connected to the same pole line,different frequency bands for the two stations.

It is to be noted that, in those cases where a frequency band comprisesone single frequency, this must not necessarily constitute an integermultiple of the system frequency. As mentioned above with reference tothe control equipment described in unpublished Swedish patentapplication 9700897-3, the control units described therein are arrangedfor reduction of a tone of a tone frequency nω of the ac components ofthe direct current, where n is a real number different from zero. Thepieces of control equipment according to the invention may thus, in thisembodiment, be adapted to influence the frequency content of therespective influencing quantity to comprise specific arbitraryfrequencies.

What is claimed is:
 1. A pair of first and second control equipment(CE11, CE12) for a first and a second active filter (AF11, AF12),respectively, in a bipolar converter station for conversion betweenalternating current and high-voltage direct current, said filters beingintended for reduction of harmonics in a dc link, connected to theconverter station, with a first and a second pole line (PC1, PC2),respectively, whereby each of the filters, in dependence on an actualcurrent value (SSI) formed in dependence on a sum of harmonic currentsin the respective pole line and supplied to a respective piece ofcontrol equipment, generates an influencing quantity (IF11, IF12) andsupplies this to the dc link, the first filter on that part of the dclink which comprises the first pole line and the second filter on thatpart of the dc link which comprises the second pole line, characterizedin that each of the pieces of control equipment comprisesfrequency-selecting means (BP, CTR) which influence the frequencycontent of the respective influencing quantity to lie essentially withinone of a first and a second of at least two predetermined, mutuallynon-overlapping frequency bands (FR1, FR2) and that, during bipolaroperation of the converter station, said frequency-selecting means inthe control equipment for the first active filter influence thefrequency content of the influencing quantity, generated by this filter,to lie within said first frequency band and said frequency-selectingmeans in the control equipment for the second active filter influencethe frequency content of the influencing quantity, generated by thisfilter, to lie within said second frequency band.
 2. A pair of controlequipment according to claim 1, characterized in that saidfrequency-selecting means in the control equipment for the first filterinfluences the frequency content of the influencing quantity, generatedby this filter, in addition thereto to lie essentially also within athird predetermined frequency band (FR3) which does not overlap saidfirst and second frequency bands.
 3. A pair of control equipmentaccording to claim 2, characterized in that said second frequency bandis located between said first and third frequency bands.
 4. A pair ofcontrol equipment according to claim 1, characterized in that each ofsaid frequency bands essentially comprises one single frequency (nω)constituting a multiple of the system frequency (ω) for an ac networkconnected to the converter station.
 5. A pair of control equipmentaccording to claim 4, characterized in that said single frequencyconstitutes an integer multiple (k) of a product of said systemfrequency and the pulse number (p) for a converter included in theconverter station.
 6. A pair of control equipment according to claim 1,characterized in that the frequency-selecting means comprise band-passfiltering members (BP1, BP2, BP3), each one with a frequencycharacteristic related to one of said first and second frequency bands,said band-pass filtering members being arranged at a respectivecurrent-sensing member (AV11, AV12) for forming the respective actualcurrent value.
 7. A pair of control equipment according to claim 1, eachof the pieces of control equipment having a control device (CTR) and adeviation-forming unit (DEVU), characterized in that frequency-selectingmeans comprise band-pass filtering members (BP1, BP2, BP3), each onewith a frequency characteristic related to one of said first and secondfrequency bands, said band-pass filtering members being arranged at arespective deviation-forming unit.
 8. A pair of control equipmentaccording to claim 1, each of the pieces of control equipment having acontrol device (CTR) comprising at least one single-tone controller(RD1, RD2, RD3) for cancellation of a tone (nω), selectable within atleast said first and second frequency bands, characterized in that saidfrequency-selecting means consist of the single-tone controller.
 9. Afirst and second pair of control equipment (CE11, CE12 and CE21, CE22,respectively) according to claim 1, of which the first pair is intendedfor a first converter station and the second pair for a second converterstation, which are mutually connected by means of the dc link, and thefirst filters (AF11 and AF12, respectively) in the first and in thesecond converter station supply the influencing quantity, generated bythe respective filter, to that part of the dc link which comprises thefirst pole line (PC1) and the second filters (AF12 and AF22,respectively) in the first and in the second converter station supplythe influencing quantity, generated by the respective filter, to thatpart of the dc link which comprises the second pole line (PC2),characterized in that, during bipolar operation of the converterstations, the frequency-selecting means in the control equipment for thefirst active filter (AF11) in the first converter station and thefrequency-selecting means in the control equipment (AF22) for the secondactive filter in the second converter station influence the frequencycontent of the influencing quantity, generated by the respective filter,to lie essentially within the first frequency band (FR1) and that thefrequency-selecting means in the control equipment for the second activefilter (AF12) in the first converter station and the frequency-selectingmeans in the control equipment for the first active filter (AF21) in thesecond converter station influence the frequency content of theinfluencing quantity, generated by the respective filter, to lieessentially within the second frequency band (FR2).
 10. A method forreducing harmonics in a dc link with a first and a second pole line(PC1, PC2) by means of a pair of a first and a second active filter(AF11, AF12), said dc link being connected to a bipolar converterstation for conversion between alternating current and high-voltagedirect current, whereinan actual current value (SSI) is formed independence on a sum of harmonic currents in the respective pole line,each of the filters, in dependence thereon, generates an influencingquantity (IF11, IF12) and supplies this to the dc link, the first filterto that part of the dc link which comprises the first pole line and thesecond filter to that part of the dc link which comprises the secondpole line, characterized in that during bipolar operation of theconverter station, the frequency content of the influencing quantitygenerated by the first filter is influenced to lie substantially withinat least one predetermined first frequency band (FR1), and the frequencycontent of the influencing quantity generated by the second filter isinfluenced to lie substantially within at least one predetermined secondfrequency band (FR2), said first and second frequency bands beingmutually non-overlapping.
 11. A method according to claim 10,characterized in that the frequency content of the influencing quantity,generated by the first filter, is influenced to lie, in additionthereto, also within a predetermined third frequency band (FR3) notoverlapping said first and second frequency bands.
 12. A methodaccording to claim 11, characterized in that the frequency content ofthe influencing quantity, generated by the second filter, is influencedto lie substantially within a second frequency band which is locatedbetween said first and third frequency bands.
 13. A method according toclaim 10, characterized in that the frequency content of the influencingquantities, generated by the filters, are influenced to consist of tones(nω) constituting multiples of the system frequency (ω) for an acnetwork connected to the converter station.
 14. A method according toclaim 13, characterized in that said tones constitute integer multiples(p) of a product of said system frequency and the pulse number (p) for aconverter included in the converter station.
 15. A method according toclaim 10, characterized in that the frequency content of the influencingquantities, generated by the filters, are influenced by means offiltering of the respective actual current values.
 16. A methodaccording to claim 10, characterized in that the frequency content ofthe influencing quantities, generated by the filters, are influenced bymeans of a control device (CTR), comprising at least one single-tonecontroller (RD1, RD2, RD3) for cancellation of a tone (nω) which isselected within at least said first and second frequency bands.
 17. Amethod according to claim 10 for reducing harmonics in a dc link with afirst and a second pole line (PC1, PC2) by means of a pair of activefilters (AF11, AF12 and AF21, AF22, respectively), said dc linkconnecting a first and a second converter station for conversion betweenalternating current and high-voltage direct current, wherein the firstpair is intended for the first converter station and the second pair forthe second converter station, and the first filters (AF11 and AF21,respectively) in the first and in the second converter station supplythe influencing quantity, generated by the respective filter, to thatpart of the dc link which comprises the first pole line (PC1) and thesecond filters (AF12 and AF22, respectively) in the first and in thesecond converter station supply the influencing quantity, generated bythe respective filter, to that part of the dc link which comprises thesecond pole line (PC2), characterized in that during bipolar operationof the converter stations,the influencing quantities generated by thefirst active filter (AF22) in the first converter station and the secondactive filter (AF22) in the second converter station are influenced tolie essentially within the first frequency band (FR1), and theinfluencing quantities generated by the second active filter (AF12) inthe first converter station and the first active filter (AF21) in thesecond converter station are influenced to lie essentially within thesecond frequency band (FR2).